The signal-to-noise power ratio of a photodetector is proportional to: EQU 1/[2q(i.sub.p +i.sub.d)(F.sub.e B)+4kTB/(R.sub.eq M.sup.2)](1)
where q is the electronic charge, i.sub.p is the primary photocurrent, i.sub.d is the dark current, F.sub.e is the excess noise factor, B is the bandwidth, k is Boltzmann's constant, T is the temperature, R.sub.e is the equivalent resistance of the load, and M is the gain. As one can readily appreciate from this, the signal-to-noise power ratio of the photodetector increases as the contributions of the first and second terms in the denominator of eqn (1) decrease. Further, the contribution of the first term is made smaller if the excess noise factor, F.sub.e, decreases and the contribution of the second term, representing thermal noise, is made smaller if the gain increases. Thus, in order to increase the signal-to-noise power ratio of the photodetector, it is desirable to have low noise and high gain.
In a p-n junction device, noise arises from the generation and subsequent collection of independent charge carriers. In an avalanche photodiode (APD), additional noise arises from the fluctuation in the carrier multiplication since, in general, this amplification is not fully deterministic. A. S. Tager, in an article entitled "Current Fluctuations In A Semiconductor (Dielectric) Under The Conditions Of Impact Ionization and Avalanche Breakdown," in Sov. Phy. - Solid State, Vol. 6, 1965, pp. 1919-1925, and later R. J. McIntyre, in an article entitled "Multiplication Noise In Uniform Avalanche Diodes," IEEE Trans. Electron Dev., Vol. ED-13, 1966, pp. 164-168, demonstrated that the avalanche multiplication process produces the least noise for electron multiplication when "a" is much greater than "b". Conversely, for hole multiplication, it is desirable that "b" be much greater than "a".
The following refers to prior art which disclose semiconductor photodetector devices having structures that increase the ratio of electron and hole ionization rates in the devices:
(1) An article entitled "Impact Ionization In Multilayered Heterojunction Structures," by R. Chin, N. Holonyak, G. E. Stillman, J. Y. Tang, and K. Hess, Electronics Letters, Vol. 16., 1980, pp. 467-469, discloses the use of a superlattice structure consisting of alternating thin layers of GaAs and Al.sub.x Ga.sub.1-x As in an attempt to increase the ratio of electron and hole ionization rates in a quantum well APD. In these devices, both the electron and hole ionization rates are enhanced above their respective values in the bulk material, however, the enhancement of the hole ionization rate is much less than that of the electron ionization rate.
(2) An article entitled "The Channeling Avalanche Photodiode: A Novel Ultra-Low-Noise Interdigitated p-n Junction Detector," by F. Capasso, IEEE Trans. Electron Dev., Vol. ED-29, 1982, pp. 1388-1395 discloses the use of a superlattice structure consisting of alternating n-GaAs and p-Al.sub.0.45 Ga.sub.0.55 As layers in another attempt to increase the ratio of electron and hole ionization rates in a channeling APD. The channeling APD is configured so that a reverse bias produces both transverse and longitudinal electric field components therein. The transverse field sweeps holes out of the GaAs layers and into adjacent Al.sub.0.45 Ga.sub.0.55 As layers while confining the electrons within the GaAs layers. The ensuing separation of charged carriers leads to substantially different ionization rates for the two carrier species.
(3) An article entitled "Staircase Solid State Photomultipliers And Avalanche Photodiodes With Enhanced Ionization Rate Ratio," by F. Capasso, W. T. Tsang and G. F. Williams, IEEE Trans. Electron Dev., Vol, ED-30, 1983, pp. 381-390 discloses the use of a superlattice structure whose layers have a graded energy bandgap in yet another attempt to increase the ratio of electron and hole ionization rates in a graded gap staircase APD. In such an APD, impact ionization events occur at specific and localized areas within the device, whereas, in a channeling APD impact ionization events occur randomly throughout each layer. Because impact ionization is localized in a graded gap staircase APD, the variability and, hence, the gain fluctuation of the number of electrons generated per photon is reduced. Thus, the excess noise in the graded gap staircase APD is lower than that in a conventional uniform APD and in a channeling APD. Nevertheless, a graded gap staircase APD fabricated from GaAs/AlGaAs material does not achieve the optimum low-noise condition because the conduction band edge offset between GaAs and AlGaAs is not a large enough fraction of the energy bandgap of GaAs to cause a sufficient difference in "a" and "b".
(4) An article entitled "Single-Carrier-Type Dominated Impact Ionization In Multilayer Structures," by H. Blauvelt, S. Margalit, and A. Yariv, Electronics Letters, Vol. 18, 1982, pp. 375-376, referred to hereinafter as Blauvelt et al, discloses the use of a doped superlattice structure consisting of p.sup.+ -i n.sup.+ AlGaAs layers followed by near intrinsic GaAs and AlGaAs layers in still yet another attempt to increase the ratio of electron and hole ionization rates in a quantum well APD. This structure is aimed at increasing the effective difference between "a" and "b" by presenting a short region of high electric field. This field "launches" electrons into GaAs with a velocity that makes ionization probable, but launches holes into AlGaAs at an energy which is insufficient for ionization in that material.
(5) An article entitled "Tunable Barrier Heights And Band Discontinuities Via Doping Interface Dipoles: An Interference Engineering Technique And Its Device Applications," by F. Capasso, K. Mohammed, and A. Y. Cho, J. Vac. Sci. Technol., B3(4), Jul/Aug 1985, pp. 1245-1251, referred to hereinafter as Capasso et al, discloses the use of a doped superlattice structure consisting of i-p.sup.+ -i GaAs layers followed by i-n.sup.+ -i AlGaAs layers in still yet another attempt to increase the ratio of electron and hole ionization rates in a quantum well APD (a similar structure is also disclosed in an InP/GaInAs material system). This structure is intended to have the same effect as that described for the device disclosed in Blauvelt et al but it differs in that it consists of p.sup.+ - and n.sup.+ -doped regions which are disposed on opposite sides of a GaAs-AlGaAs heterojunction.
(6) A patent application entitled "Avalanche Photodetector," by Kevin F. Brennan, Ser. No. 894,004, Filed Aug. 7, 1986, discloses the use of a doped superlattice structure consisting of p.sup.+ -n.sup.+ AlGaAs layers followed by near intrinsic GaAs and AlGaAs layers in still yet again another attempt to increase the ratio of electron and hole ionization rates in a quantum well APD. This structure, like the structure disclosed in Blauvelt et al, is designed to spatially restrict the regions wherein impact ionization occurs in order to minimize the variability of the number of electrons generated per detected photon. Unfortunately, the devices disclosed in the Blauvelt et al article and in the Brennan patent application are difficult to fabricate because it is very difficult to control the n-type dopant in the AlGaAs material. Specifically, at the temperatures used to grow the AlGaAs alloy layers, the n-type dopant is very mobile and tends to be transported by the growth interface rather than staying in place at the location at which it was first deposited. This behavior of n-type dopants in AlGaAs is different from the behavior of n-type dopants in GaAs (which is grown at lower temperature). Further, attempts to grow AlGaAs layers at lower temperatures so that n-type dopants would remain fixed therein, have generally produced poor quality AlGaAs layers.
In light of the above, a need exists for a low noise, high gain APD.